How a cochlear implant works

Recently I was asked how a Cochlear Implant works. First of all if you understand how normal hearing works it might help you to understand how the cochlear implant works. (I'm not a doctor nor an audiologist, simply an implantee! so this is not a medical treatise!)

(1) Understanding how our ear processes sound
Sound around us produces vibrations of varying degrees (frequencies). Our ears pick up these vibrations, channel them to the middle ear which enhances the vibrations before they travel to our cochlea. In our cochlea the vibrations cause a membrane to vibrate and this membrane activates 1,000s of hair cells. These hair cells translate the vibrations into electrical impulses which travel up our hearing nerve into our brain where we process the impulses as sound.

(2) What happens in deafness
The most common cause (but not every cause) of deafness is sensorineural deafness. What this means is those little hair cells in the cochlea start dying off (most often the high pitched frequencies first). So it doesn't matter how loud a sound is we can never hear it because the hair cells aren't there so we can't create that electrical impulse needed to understand sound. In effect a connection is missing. We are still receiving the vibrations through our ears, and our hearing nerve and brain are still working ... but the hair cells are missing (the connection between our cochlea and hearing nerve) and we can't produce that electrical impulse.

(3) A hearing aid
A hearing aid cannot replace those missing hair cells nor create the electrical impulse we need to hear sound. A hearing aid simply amplifies the sound and while this helps if you can still hear that frequency (ie the hair cells needed to make the electrical impulse for that frequency are still working) a hearing aid cannot make up for sounds we cannot hear. So a hearing aid can sometimes be a hindrance because it amplifies some frequencies while missing out on others (because we can’t hear them). This can make it particularly difficult to hear in background noise. When the dull roar is amplified it makes it even harder to hear higher pitched sounds such as speech.

(4) Cochlear Implant and Speech processor
A cochlear implant consists of two parts (1) The internal implant positioned by a surgeon and (2) The external Speech Processor. You need both parts to hear. The speech processor looks a bit like a hearing aid but it is nothing like a hearing aid because it does not amplify sound. The processor is four mini computers which turn sound vibrations into electrical impulses. These impulses travel to the external coil and communicate with the internal implant. The implant then sends these electrical impulses to the cochlea and stimulates the hearing nerve through the cochlear wall. So in effect, the cochlear implant by-passes the damaged hair cells, providing the connection that is missing. In other words the cochlear implant makes the electrical impulses we can no longer make ourselves and therefore we hear frequencies (sounds) we cannot naturally hear when our hair cells have died. Naturally we cannot feel these electrical impulses any more than you can feel them now with the hearing you have remaining.

Once the hearing nerve is stimulated by the electrical impulses our natural hearing takes over, the impulses travel along the hearing nerve to our brain and it is here we interpret and understand sound.

How well we understand the new electrical impulses stimulating our cochlear depends on how long we’ve been deaf, whether we ever had good hearing, whether we have an underlying medical condition and so on.